1. Field of the Invention
The present invention relates to a power supply, and more particularly to a power supply having a two-way DC to DC converter.
2. Description of Related Art
Portable electronic devices such as notebook computers, Personal Digital Assistants (PDAs) or the like are commonly used because they are convenient for users to carry and use. A critical drawback of the portable electronic devices is that the lifetime of batteries in the portable electronic devices is short. Before the batteries are running out of power, the users have to connect the portable electronic devices to utility power through a power supply to maintain normal the portable electronic devices operating.
A conventional power supply having additional power output and input ports is existed. The additional power input port is used to obtain a substitute DC power when the utility power is unavailable. The additional power output port additionally outputs a DC power to another electronic device other than the electronic device using the conventional power supply. With reference to FIGS. 11 and 12, the conventional power supply may be used with a notebook computer and a Universal Serial Bus (USB) device and comprises a AC to DC converter (51), a first DC to DC converter (52) and a second DC to DC converter (53). The AC to DC converter (51) comprises an input terminal (511), a full-wave rectifier (512), a power factor correction (PFC) circuit (513), a fly-back DC to DC converter (514) and an output terminal (515).
The AC to DC converter (51) receives an AC power such as utility power from the input terminal (511), rectifies and filters the AC power, converts the AC power to a middle level DC power such as 19 volts DC power and outputs the middle level DC power from the output terminal (515). The notebook computer obtains the middle level DC power as the operating power when the notebook computer is electrically connected to the output terminal (515) of the AC to DC converter (51). The first DC to DC converter (52) has an input terminal (521) and an output terminal (522). The input terminal (521) of the DC to DC converter (52) is connected to the output terminal (515) of the AC to DC converter (51) to obtain and convert the middle level DC power to a low level DC power. The output terminal (522) of the first DC to DC converter (52) outputs the low level DC power when the USB device is electrically connected to the output terminal (522) so the USB device obtains operating power. The second DC to DC converter (53) has an output terminal (531) and an input terminal (532). The output terminal (531) of the second DC to DC converter (53) is connected to the output terminal (515) of the AC to DC converter (51). The input terminal (532) of the second DC to DC converter (53) is connected to an external DC power maybe from a vehicle cigarette lighter at 12 to 16 volts. The second DC to DC converter (53) converts the external DC power to the middle level DC power. The output terminal (515) of the AC to DC converter (51) outputs the middle level DC power. The middle level DC power can also be converted to the low level DC power by the first DC to DC converter (52), and the output terminal (522) of the first DC to DC converter (52) outputs the low level DC power.
However, the conventional power supply has drawbacks as follow.
1. The conventional power supply has big size because the conventional power supply has two DC to DC converters (52, 53). More DC to DC converters (52, 53) also results in more power consumption.
2. When stay in an environment without utility power such as on vehicles, the users do not often use both the notebook computer and the USB device. In particular, when the conventional power supply obtains the external DC power rather than the AC power, the conventional power supply is mostly used to supply power to the notebook computer rather than the USB device so the first DC to DC converter (52) is ineffectual.
To overcome the shortcomings, the present invention provides a power supply having a two-way DC to DC converter to mitigate or obviate the aforementioned problems.